1. Field of the Invention
The present invention relates to a device for detecting a combustion condition or misfire of an internal combustion engine.
2. Description of the Prior Art
Various ignition systems for use in multi-cylinder internal combustion engines are known, for example, as shown in FIG. 4A, there is known a distributor type ignition system which includes an ignition coil 50, a power transistor 52 for making battery current flow through a primary winding 50a of the ignition coil 50, an engine control unit (ECU) 54 for driving the power transistor 52 in sequence and in timed relation to the ignition timings of each cylinder #1.about.#4 and inducing a high voltage for ignition in a secondary winding 50b of the ignition coil 50, and a distributor 55 for distributing the high voltage for ignition to spark plugs 56.about.59 of the respective cylinders #1.about.#4 of the internal combustion engine sequentially, whereby the ignition system is adapted to distribute the high voltage for ignition to each spark plugs by way of the distributor 55.
As shown in FIG. 4b, there is further known a single-ended distributorless ignition system which includes a plurality of ignition coils 61 and 62 corresponding to each cylinders #1 and #2 of an internal combustion engine, power transistors 64 and 65 for making battery current flow through primary windings 61a and 62a of the ignition coils 61 and 62, and an engine control unit (ECU) 67 for driving the power transistors 64 and 65 one by one and in timed relation to the ignition timings of each cylinders #1 and #2 and inducing a high voltage for ignition in secondary windings 61b and 62b of the ignition coils 61 and 62, whereby the ignition system is adapted to apply a high voltage for ignition produced at each secondary windings 61b and 62b to each spark plugs 68 and 69.
Though not shown, there is further known a double-ended distributorless ignition system which is constructed so as to make a secondary winding of an ignition coil be connected at opposite ends thereof to a pair of spark plugs provided to different cylinders and thereby be capable of applying a high voltage for ignition from one ignition coil to two spark plugs simultaneously.
In each of such ignition systems, there is normally incorporated a combustion condition or misfire detecting device which is adapted to detect a combustion condition or misfire of each cylinders of an internal combustion engine on the basis of a waveform of a voltage obtained after spark discharge of the spark plug.
For example, the distributor type ignition system shown in FIG. 4A is provided with a misfire detecting device which consists of a voltage dividing circuit 78 made up of coupling capacitors 71.about.74 of a small capacity, disposed in a conductive path for applying a high voltage for ignition to the spark plugs 56.about.59 and capacity, disposed in a conductive path for applying a high voltage for ignition to the spark plugs 56.about.59 and a capacitor 76 of a relatively large capacity and a resistor 77 which are connected to the coupling capacitors 71.about.74 at one end and grounded at another end, respectively, and a misfire detecting circuit 80 for detecting a misfire of each cylinders #1.about.#4 on the basis of a decay characteristic of a divided voltage which is obtained by means of the voltage dividing circuit 78 after ignition or firing of each cylinders #1 .about.#4. Further, the single-ended distributorless ignition system is provided with a misfire detecting device which consists of a moltage dividing circuit made up of capacitors 81 and 82 of a small capacity, a capacitor 84 of a relatively large capacity and a resistor 85, and a misfire detecting circuit 87 for detecting a misfire of each cylinders #1 and #2 on the basis of a decay characteristic of a divided voltage obtained by means of the voltage dividing circuit.
However, in the prior art misfire detecting device, the coupling capacitor of a small capacity, constituting part of the voltage dividing circuit, is directly provided to a conductive path (i.e., high tension code) for each spark plug, to which a high voltage for ignition is applied, in order to detect a voltage waveform obtained after spark discharge. Accordingly, it requires coupling capacitors, each of which is of a high withstand voltage and expensive as it goes, by the number corresponding to that of cylinders, thus causing a problem of a high cost. Further, in order to fix the coupling capacitors to the conductive paths (i.e., high tension codes) for the spark plugs, a fixing device only for that end is necessitated. In this connection, a plurality of such fixing devices corresponding in number to the cylinders are in effect necessitated, thus causing a problem of a high cost and a difficult assembling work.
Further, in the double-ended distributorless ignition system in which a high voltage for ignition is applied from one ignition coil to two spark plugs simultaneously, a negative high voltage is applied as a high voltage for ignition to one of the two spark plugs. In the spark plug to which a negative voltage is applied, an electrical resistance between the center electrode and the outer electrode is maintained high even in the case where normal combustion occurs, similarly to the case where a misfire has occurred, so there is caused a problem that it is impossible to correctly distinguish between normal combustion and misfire on the basis of the voltage waveform.